The present invention relates to the field of digital distance relaying for protecting an AC electric power transmission line system from faults. More specifically, the present invention relates to methods and systems for digital distance relaying involving automatically and recursively estimating postfault electrical quantities so as to relay the fault in the very first postfault electrical AC cycle.
In the field of electrical power engineering, generating systems for producing the electric power are interconnected in a complex power grid by high voltage alternating current (AC) three-phase electric power transmission lines. Occasionally, a transmission line is faulted when a conductor wire breaks and falls to the ground or when conductors short-circuit together. The power grid is provided with circuit breakers for disconnecting a faulted section of transmission line. When properly controlled by a distance-relaying computer, the faulted section, and only that section, should be swiftly disconnected so as to avoid unnecessary interruptions of service to electric power consumers and prevent a power blackout from extending over an unnecessarily large geographic region.
Electric power systems increasingly employ more expensive, higher capacity generation and transmission equipment involving increasingly higher voltages and currents. This trend in the technology is putting pressure on the art of digital distance relaying to relay the fault as early as possible in the first AC cycle after the inception of the fault. Unfortunately, swiftness in relaying a fault has proven to be a priority which competes with the goal of accurately and confidently identifying which transmission line section is the faulted section. A need is evident for a new relaying approach to provide a greater swiftness of relaying with the same accuracy of identification.
Heretofore, line currents and voltages from the three phases of a transmission line have been successively sampled and the inception of a fault has been detected by noting unusual departure from the expected values. After the fault occurs, postfault voltage and current samples have been taken, but these are observed to be corrupted by noise and transient behavior in the first postfault cycle which has limited the speed with which the steady-state sinusoidal waveform can be identified, from which accurate impedance calculations can be made. The sooner that accurate impedance calculations can be made, the sooner the faulted line section can be accurately identified.